The present invention relates in general to the process of powder injection molding (PIM), and, in particular, to a new and useful process for removing binder from a molded powder compact in two stages.
The technology of injection molding plastics has advanced to a state of precision where plastic components having very complex shapes can be formed economically. In its simplest form, injection molding involves heating a plastic to a temperature where it can flow and then forcing the plastic into a shaped cavity where it is cooled. While this is similar to metal casting, greater precision and finer surface detail are possible for injection molded plastic objects. These objects are structurally inferior in comparison with objects made of metal, however.
The injection molding of filled polymers with dispersed metallic or ceramic powders is a recognized technique for improving the strength of the polymers. Filled polymers are composites which incorporate the ease of fabrication of a polymer and the mechanical, thermal, magnetic and electrical properties of the filler. Until recently, the highest concentrations of fillers were in magnetic materials where the polymer served the role of a binder. See R. P. Kusy, "Applications," Metal-Filled Polymers, S. K. Bhattacharya (ed.), Marcel Dekker, New York, NY, 1986, pp. 1-142.
A recent development in this field is to maximize the content of solid particles and to actually remove the polymer binder during a sintering operation. As a consequence, a new powder forming process has evolved which permits the complexity of shape and low cost of injection molded plastics, while having high performance properties characteristic of metals, ceramics, cermets (ceramic plus metal) and other composites. This new process is termed powder injection molding (PIM).
The steps involved in forming a component by PIM, include:
1. Selection and tailoring of a powder for the process; PA0 2. Mixing the powder with a suitable binder; PA0 3. Production of homogeneous granular pellets of mixed powder plus binder; PA0 4. Forming a part by injection molding in a closed die; PA0 5. Processing the formed part to remove the binder (termed debinding); PA0 6. Densification of the compact by high temperature sintering; and PA0 7. Post-sintering processing as needed, including heat treatment, further densification or machining. PA0 A. R. Erickson and H. E. Amaya, "Recent Developments in Injection Moulding of PM Parts," Metal Powder Report, 1985, vol. 40, pp. 484-489; PA0 R. Billet, "Plastic Metals; The Injection Molded P/M Materials Are Here," Proceedings P/M-82, Associazione Italiana di Metallurgia, Milano, Italy, 1982, pp. 603-610. PA0 H. Reh, "High Performance Ceramics I. Definition and Economic Significance," Powder Metallurgy International, 1986, vol. 18, pp. 84-86; and PA0 A. R. Erickson and R. E. Wiech, Jr., "Injection Molding," Metals Handbook, Vol. 7, ninth edition, American Society for Metals, Metals Park, OH, 1984, pp. 495-500. PA0 R. J. Walkar and B. R. Patterson, "P/M Injection Molding," Horizons of Powder Metallurgy, Part II, W. A. Kaysser and W. J. Huppmann (eds.), Veriag Schmid, Freiburg, West Germany, 1986, pp. 661-665; PA0 R. Billet, "Plastic Metals; From Fiction to Reality with Injection Molded P/M Materials," Progress in Powder Metallurgy, 1982, vol. 38, pp. 45-52; and PA0 R. Billet, "Net-Shape Full Density P/M Parts by Injection Molding," International Journal of Powder Metallurgy and Powder Technology, 1985, vol. 21, pp. 119-129.
The binder may be thermoplastic polymeric material, water or various inorganic substances. These have all been used successfully in PIM. The amount of binder ranges from 15% to 50% by volume of the mixture. The particles tend to be small in size to aid in the sintering densification step, although this proves expensive and sometimes produces problems with flow into the die. Some progress has been made using powders as large as 100 micrometers. The injection molding step is similar to that used for molding conventional polymeric materials and involves concurrent heating and pressurization cycles. After molding, binder removal from the powder compact can be a slow step and a source of problems. Various thermal, solvent and capillary extraction approaches are in use. By sintering, void spaces remaining after removal of the binder are eliminated with commensured shrinkage. Isotropic powder packing allows for predictable and uniform shrinkage. The dies used in PIM are thus generally over-sized to accomodate for the final compact dimensions and powder packing density. The resulting compact has micro structural homogeneity and isotropic properties which are superior to those available with many other processes. Generally, the sintering density for an item made by PIM exceeds 93% of theoretical as compared with less than 85% density, that is often attained in die compacted materials. Furthermore, the pores that do remain in PIM compacts are not interconnected or are small spherical spores.
These cause less detriment to the properties of the compact and, consequently, the properties of PIM compacts are very attractive.
These favorable characteristics are further enhanced by the economical and versatile nature of PIM.
References that discuss powder injection molding processes include:
The economical advantages of PIM are disclosed in A. R. Erickson and H. E. Amaya, "Recent Developments in Injection Moulding of PM Parts," Metal Powder Report, 1985, Vol. 40, pp. 484-489; and B. Williams, "Current Status of Injection Molding of Metal Powders and Ceramics," Metal Powder Report, 1986, Vol. 41, pp. 359-364.
Some of the superior properties of PIM products are disclosed in:
U.S. Pat. No. 4,713,206 to Matsuhisa et al. discloses the use of a rigid ceramic body having an open structure for supporting a casting during a de-waxing step for receiving binder from the casting. It is essential in this reference to carefully and accurately machine the surface of the ceramic body which receives the casting. Only relatively slow heating rates of not more than 100.degree. C. per hour are permitted and preferably not more than 10.degree. C. per hour. Japanese patent 61-77671 is relevant for its showing of a defatting process for a ceramic body which apparently is embedded in a heat resistant powder. A single step process is disclosed and, while a purpose of the reference is to reduce defatting time, it is not clear whether accelerated heating rates are possible.
The following U.S. patents are also relevant to the PIM process: U.S. Pat. Nos. 4,113,480 to Rivers and 4,197,118, 4,305,756 and 4,404,166, all to Wiech.